The current and next-generation wireless communication systems will utilize improved PA efficiency technology for a variety of broadband and multimedia services, supported by the advanced wireless RF transmitter, in which the linear and high-efficient high power amplifier will improve the transmitted quality, system performance and cost saving. Traditional predistortion technologies and schemes used widely to linearize PA in wireless communication systems are (i) analog feed-forward linearizers implemented in RF band by means of analog hardware circuit, and (ii) digital predistortion schemes in the baseband that involve a feedback channel and driven by digital signal processing (DSP) algorithms and integrated circuits.
Conventional analog predistortion schemes are mainly based on the principle of error subtraction and power-matching with dedicated hardware circuitries to realize non-linear corrections to PA. These approaches must use an auxiliary PA and complicated hardware circuitries to match exactly the transmitted power-balance, time-delay and errors generated by the main PA. After a perfect matching is obtained, the non-linear distortion errors from the main PA can then be canceled by those distortion errors from the auxiliary PA. Due to the complexities of the nonlinear predistortion circuits, which among other things involve many variables and parameters, the analog schemes require significant fine tuning and other calibration efforts. In addition, such traditional analog schemes are also vulnerable to fluctuating environmental conditions, such as temperature and humidity changes, since perfect alignments of the main PA's signal and that of the auxiliary PA are vital. As a result, traditional predistortion schemes are costly to implement and are limited in their predistortion accuracy and stability in commercialized wireless system environment.
Conventional DSP-based digital predistortion schemes utilize digital microprocessors to compute, calculate and correct PA's nonlinearities: they perform fast tracking and adjustments of signals in the PA system. Since the computations are performed in the digital domain, such digital schemes can also accommodate a wider fluctuations of environmental conditions, and reduce the extent of fine-tuning or calibrations during the manufacturing stage. However, traditional digital predistortion schemes necessitate coded in-phase (I) and quadrature (Q) channel signals in the baseband as the required ideal or reference signals. As a result, the feedback signal from PA output must be down-converted to baseband area by the arrangement of down-converting and demodulation circuitries. Therefore, in order to deploy traditional digital predistortion schemes into base-stations, the digital predistortion engines must be embedded into the baseband architecture of base-stations. This embedment is a practical implementation challenge since it is frequently inconvenient or impossible to modify the baseband architectures of existing base-stations or base-station designs. Furthermore, since traditional digital predistortion approaches require baseband I-Q signal sources to operate, they are inapplicable to certain RF systems that do not possess any baseband I-Q signal sources, such as repeater and indoor signal coverage sub-systems.